<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    * {
      margin: 0;
      padding: 0;
    }

    .box {
      position: relative;
      width: 1226px;
      height: 600px;
      background: url('./images/bg.jpg');
      margin: 0 auto;
    }

    .son {
      /* 子绝父相 */
      position: absolute;
      left: 0;
      bottom: 0;

      /* 注意点：绝对定位的元素会脱标，脱标之后的元素宽高默认由内容撑开 */
      width: 100%;
      height: 200px;
      background-color: rgba(0,0,0,.5);
    }

  </style>
</head>
<body>
  <div class="box">
    <div class="son">我是内容我是内容我是内容</div>
  </div>
</body>
</html>